[lldb] [Plugins/FreeBSDRemote] Disable GetMemoryRegionInfo()

Disable GetMemoryRegionInfo() in order to unbreak expression parsing.
For some reason, the presence of non-stub function causes LLDB to fail
to detect system libraries correctly.  Through being unable to find
mmap() and allocate memory, this leads to expression parser being
broken.

The issue is non-trivial and it is going to require more time debugging.
On the other hand, the downsides of missing the function are minimal
(2 failing tests), and the benefit of working expression parser
justifies disabling it temporarily.  Furthermore, the old FreeBSD plugin
did not implement it anyway, so it allows us to switch to the new plugin
without major regressions.

The really curious part is that the respective code in the NetBSD plugin
yields very similar results, yet does not seem to break the expression
parser.

Differential Revision: https://reviews.llvm.org/D90650