[gdb/testsuite] Remove stale exec in gdb_compile_ada

When running test-case gdb.ada/ptype_tagged_param.exp, I get:
...
PASS: gdb.ada/ptype_tagged_param.exp: compilation foo.adb
...

However, when I then de-install gnatmake and run again, I get the same result.
This is due to a stale exec. After removing the stale exec, I get:
...
UNSUPPORTED: gdb.ada/ptype_tagged_param.exp: compilation foo.adb
...

Fix this removing the stale exec in gdb_compile_ada before compilation.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-13  Tom de Vries  <tdevries@suse.de>

	* lib/ada.exp (gdb_compile_ada): Delete stale exec before compilation.