gdb/testsuite: Default to assembler's preferred debug format in asm-source.exp

The stabs debug format is obsolete and there's no reason to think that
toolchains still have good support for it. Therefore, if a specific debug
format wasn't set in asm-source.exp then leave it to the assembler to
decide which one to use.

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