gdb/dwarf: remove unnecessary compunit_symtab_set_p check in dw2_do_instantiate_symtab

dw2_do_instantiate_symtab is called by dw2_instantiate_symtab, only if
per_cu does not have a matching compunit_symtab yet.
dw2_do_instantiate_symtab therefore does not need this
compunit_symtab_set_p itself.

Change-Id: Ie665980bb65016b56206c4aac36f4c2fc13c61de
Approved-By: Tom Tromey <tom@tromey.com>