[lldb] Remove linux/mips debugging support

As discussed on lldb-dev
<https://lists.llvm.org/pipermail/lldb-dev/2021-March/016777.html> the
mips code is unmaintained and untested. It also carries a lot of
technical debt which is not limited to mips-specific code.

Generic mips support remains (and is going to be used by the upcoming
freebsd code). Resurrecting mips support should be a matter of re-adding
the relevant register context files (while avoiding reintroducing the
debt).

Author: labath

lldb/source/Plugins/Process/Linux/CMakeLists.txt

@@ -3,7 +3,6 @@ add_lldb_library(lldbPluginProcessLinux
   NativeRegisterContextLinux.cpp
   NativeRegisterContextLinux_arm.cpp
   NativeRegisterContextLinux_arm64.cpp
-  NativeRegisterContextLinux_mips64.cpp
   NativeRegisterContextLinux_ppc64le.cpp
   NativeRegisterContextLinux_s390x.cpp
   NativeRegisterContextLinux_x86_64.cpp