[mlir] Drop customization hooks from StandardToLLVM conversion

Summary:
These hooks were originally introduced to support passes deriving the
StandardToLLVM conversion, in particular converting types from different
dialects to LLVM types in a single-step conversion. They are no longer in use
since the pass and conversion infrastructure has evolved sufficiently to make
defining new passes with exactly the same functionality simple through the use
of populate* functions, conversion targets and type converters. Remove the
hooks. Any users of this hooks can call the dialect conversion infrastructure
directly instead, which is likely to require less LoC than these hooks.

Differential Revision: https://reviews.llvm.org/D73795