[lldb] Remove redundant register alt_names

Remove redundant register alt_names that correspond to their respective
generic names.  D108554 makes it possible to query registers through
their generic names directly, therefore making repeating them via
alt_name unnecessary.

While at it, also remove alt_names that are equal to register names
on PPC.

This patch does not alter register definitions where the generic names
are listed as primary names, and other names are provided as alt_name
(e.g. ARM).

Differential Revision: https://reviews.llvm.org/D109626