[lldb/gdb-remote] Remove ancient debugserver workaround

This workaround is the source of an awkwared Process->Platform
dependency. While this could be solved in various ways (the only thing
we really use is the plugin name), it may be better to just remove it --
the workaround was added 10 years ago (43c555df), and the affected
debugservers were "old" even then, so hopefully they are not in use
anymore.

Differential Revision: https://reviews.llvm.org/D121305