[libc++][NFCI] Remove unnecessary exception-throwing base classes

__split_buffer_common was entirely unused, and __deque_base_common
was unused except for two calls to __throw_out_of_range(), which have
been inlined.

The usual intent of the __xxx_base_common base classes is to localize
where the exception-throwing code is instantiated, however that wasn't
the case here because we never explicitly instantiated those base classes
in the shared library, unlike what we do for basic_string and vector.

Differential Revision: https://reviews.llvm.org/D108384