gdb, compile: unlink objfile stored in module

gdb, compile: unlink objfile stored in module

When cleaning up after a compile command, we iterate over all objfiles and
unlink the first objfile with the same name as the one we compiled.

Since we already store a pointer to that objfile in the module and use it
to get the name we're comparing against, there's no reason to iterate, at
all.  We can simply use that objfile.

This further avoids potential issues when an objfile with the same name is
loaded into a different linker namespace.