[clang-tidy] Remove gsl::at suggestion from cppcoreguidelines-pro-bounds-constant-array-index

Currently the fix hint is hardcoded to gsl::at(). This poses
a problem for people who, for a number of reasons, don't want
or cannot use the GSL library (introducing a new third-party
dependency into a project is not a minor task).

In these situations, the fix hint does more harm than good
as it creates confusion as to what the fix should be. People
can even misinterpret the fix "gsl::at" as e.g. "std::array::at",
which can lead to even more trouble (e.g. when having guidelines
that disallow exceptions).

Furthermore, this is not a requirement from the C++ Core Guidelines.
simply that array indexing needs to be safe. Each project should
be able to decide upon a strategy for safe indexing.

The fix-it is kept for people who want to use the GSL library.

Differential Revision: https://reviews.llvm.org/D117857