[lldb] [MainLoop] Remove redundant termination clause (NFCI)

Remove the redundant termination clause from within the loop.  Since
the check is done at the end of the loop, it's entirely redundant
to the 'while' condition.  If termination was requested, the latter
will become false and the 'while' loop will terminate, resulting
in the 'return' statement below the loop being executed (which is
equivalent to the one used inside 'if').

Differential Revision: https://reviews.llvm.org/D58227

llvm-svn: 354050