gdb: Remove stabs support for COFF files

gdb: Remove stabs support for COFF files

This commit continues the removal of stabs by removing support from coff
inferiors.  This is trivial for the most part, just a removal of code
setting things only relevant for stabs, with one exception.

The global variables symnum and within_function were introduced to
coffread.c (and within_function was converted to boolean).  I looked into
making them parameters to the relevant function, but this would require
changes to several otherwise untouched functions, so I kept them as globals
instead.

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