Eliminate gdb_is_target_remote / gdb_is_target_native & friends

After the previous patches, gdb_is_target_remote,
gdb_is_target_native, and mi_is_target_remote aren't used anywhere.
This commit eliminates them, along with now unnecessary helpers.

Change-Id: I54f9ae1f5aed3f640e5758731cf4954e6dbb1bee
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed