[gdb/symtab] Remove superfluous handling of Ada main in write_cooked_index

I filed PR29179 about the following FAIL in test-case
gdb.ada/O2_float_param.exp with target board cc-with-gdb-index:
...
(gdb) break increment^M
Function "increment" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.ada/O2_float_param.exp: scenario=all: gdb_breakpoint: \
  set breakpoint at increment
...

The FAIL was a regression since commit 2cf349be ("Do not put linkage names
into .gdb_index").

Before that commit we had:
...
$ readelf -w foo > READELF
$ grep callee.*increment READELF
[1568] callee__increment: 5 [global, function]
[3115] callee.increment: 5 [global, function]
...
but after only:
...
$ grep callee.*increment READELF
[3115] callee.increment: 5 [global, function]
...

The regression was fixed by commit 67e83a0d ("Fix regression in
c-linkage-name.exp with gdb index"), which got us again:
...
$ grep callee.*increment READELF
[1568] callee__increment: 5 [global, function]
[3115] callee.increment: 5 [global, function]
...

The commit however did not claim that particular PR.  A subsequent commit,
commit 5fea9794 ("Improve Ada support in .gdb_index") did claim to fix it,
together with commit dd05fc70 ("Change .gdb_index de-duplication
implementation").

The commit 5fea9794 contained the following addition in write_cooked_index:
...
+      if (entry->per_cu->lang () == language_ada)
+	{
+	  /* We want to ensure that the Ada main function's name
+	     appears verbatim in the index.  However, this name will
+	     be of the form "_ada_mumble", and will be rewritten by
+	     ada_decode.  So, recognize it specially here and add it
+	     to the index by hand.  */
+	  if (entry->tag == DW_TAG_subprogram
+	      && strcmp (main_for_ada, name) == 0)
+	    {
+	      /* Leave it alone.  */
+	    }
+	  else
+	    {
+	      /* In order for the index to work when read back into
+		 gdb, it has to use the encoded name, with any
+		 suffixes stripped.  */
+	      std::string encoded = ada_encode (name, false);
+	      name = obstack_strdup (&symtab->m_string_obstack,
+				     encoded.c_str ());
+	    }
+	}
...

The code contains some special handling related to the Ada main function, so
let's look at that one: foo.  Before commit 67e83a0d we have:
...
$ grep foo.*function READELF
[3733] foo: 7 [global, function]
...
and after:
...
$ grep foo.*function READELF
[2738] _ada_foo: 7 [global, function]
[3733] foo: 7 [global, function]
...
so that looks identical to the callee.increment case.

At commit 5fea9794, we have slightly different index numbers:
...
$ grep foo.*function READELF
[1658] foo: 7 [global, function]
[2738] _ada_foo: 7 [global, function]
...
but otherwise the same result.

If we disable the special handling of the Ada main function like so:
...
-	  if (entry->tag == DW_TAG_subprogram
+	  if (false && entry->tag == DW_TAG_subprogram
...
we still have the exact same result because:
...
(gdb) p main_for_ada
$1 = 0x352e6a0 "_ada_foo"
...
and ada_encode ("_ada_foo", false) == "_ada_foo".

The comment seems to be copied from debug_names::insert, which does indeed use
ada_decode, while the code in write_cooked_index uses ada_encode instead.

Remove the superfluous special handling of Ada main in write_cooked_index.

Tested on x86_64-linux, with target boards unix and cc-with-gdb-index.

Approved-By: Tom Tromey <tom@tromey.com>