gdb: remove filename arg from gdb_bfd_open_from_target_memory

gdb: remove filename arg from gdb_bfd_open_from_target_memory

The filename argument to gdb_bfd_open_from_target_memory was never
used; this argument had a default value of nullptr, and the only call
to this function, in jit.c, relied on the default value.

In the next commit I'm going to make some changes to the
gdb_bfd_open_from_target_memory function, and, though I could take
account of a filename parameter, it seems pointless to maintain an
unused argument.

This commit removes the filename argument.

There should be no user visible changes after this commit.