[lldb] Remove "dwarf dynamic register size expressions" from RegisterInfo

These were added to support some mips registers on linux, but linux mips
support has now been removed due.

They are still referenced in the freebds mips implementation, but the
completeness of that implementation is also unknown. All other
architectures just set these fields to zero, which is a cause of
significant bloat in our register info definitions.

Arm also has registers with variable sizes, but they were implemented in
a more gdb-compatible fashion and don't use this feature.

Differential Revision: https://reviews.llvm.org/D110914