Fix coff_start_symtab resource leak found by Coverity

This commit fixes a resource leak found by Coverity, where
coff_start_symtab performs an xstrdup that is now performed
within start_symtab by buildsym_compunit::buildsym_compunit.

gdb/ChangeLog:

	* coffread.c (coff_start_symtab): Remove unnecessary xstrdup.