[clang-tidy][NFCI] Simplify bugprone-sizeof-expression (#93024)

This commit eliminates a redundant matcher subexpression from the
implementation of the "sizeof-pointer-to-aggregate" part of the
clang-tidy check `bugprone-sizeof-expression`.

I'm fairly certain that anything that was previously matched by the
deleted matcher `StructAddrOfExpr` is also covered by the more general
`PointerToStructExpr` (which remains in the same `anyOf`).

This commit is made to "prepare the ground" for a followup change that
would merge the functionality of the Clang Static Analyzer checker
`alpha.core.SizeofPtr` into this clang-tidy check.