gdb/xcoffread: remove xcoff_symfile_info::{symtbl,symtbl_num_syms}

It is not necessary to save the symbol table content and number of
symbols in xcoff_symfile_info, because they are only needed for the
duration of scan_xcoff_symtab.

Move the reading of the symbol table content to scan_xcoff_symtab, and
keep everything as local variables.

Change-Id: I21e2b95a0b8df2255ba46904083658a4e8cb89f0
Approved-By: Tom Tromey <tom@tromey.com>