gdb: remove special case completion word handling for filenames

This commit removes some code which is special casing the filename
completion logic.  The code in question relates to finding the
beginning of the completion word and was first introduced, or modified
into its existing form in commit 7830cf6f (from 2001).

The code being removed moved the start of the completion word backward
until a character in gdb_completer_file_name_break_characters was
found, or until we reached the end of the actual command.

However, I doubt that this is needed any more.  The filename completer
has a corresponding filename_completer_handle_brkchars function which
provides gdb_completer_file_name_break_characters as the word break
characters to readline, and also sets rl_completer_quote_characters.
As such, I would expect readline to be able to correctly find the
start of the completion word.

There is one change which I've needed to make as a consequence of
removing the above code, and I think this is a bug fix.

In complete_line_internal_normal_command we initialised temporary
variable P to the CMD_ARGS; this is the complete text after the
command name.  Meanwhile, complete_line_internal_normal_command also
accepts an argument WORD, which is the completion word that readline
found for us.

In the code I removed P was updated, it was first set to WORD, and
then moved backwards to the "new" start of the completion word.

But notice, the default for P is the complete command argument text,
and only if we are performing filename completion do we modify P to be
the completion word.

We then passed P through to the actual commands completion function.

If we are doing anything other than filename completion then the value
of P passed is the complete argument text.

If we are doing filename completion then the value of P passed is the
completion word.

In filename_completer we get two arguments TEXT and WORD, the TEXT
argument is the value of P which is the "new" completion word, while
WORD is the completion word that readline calculated.

After simplifying complete_line_internal_normal_command, and the
temporary P is removed, we always pass the complete argument text into
TEXT, while WORD remains the completion word that readline found.

Previously in filename_completer we actually tried to generate
completions based on TEXT, which worked fine as TEXT actually
contained the completion word that we found in
complete_line_internal_normal_command.  But I believe that we should
be fine to use the completion word that readline found, so I have
updated filename_completer to generate completions based on WORD.

If I'm correct, then I don't expect to see any user visible changes
after this commit.