Now that all places using gdb::string_view have been updated to use
std::string_view, this patch drops the gdb::string_view implementation
and the tests which came with it.
As this drops the unittests/string_view-selftests.c, this also
implicitly solves PR build/23676, as pointed-out by Tom Tromey.
Change-Id: Idf5479b09e0ac536917b3f0e13aca48424b90df0
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23676