gdb: remove exact_match parameter from find_line_symtab

struct symtab *find_line_symtab (struct symtab *, int, int *, bool *);

The last parameter is bool* that when set will receive information
if the match was exact. This parameter is never used by any callsite
and can therefore be removed.

This will become:

symtab *find_line_symtab (symtab *sym_tab, int line, int *index);

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