gdb: simplify find_symbol_at_address

This commit further simplifies find_symbol_at_address by moving
the special handling in case of -readnow down to readnow_functions.

Running all tests in gdb.rust with and without -readnow shown no
regressions (tested on Debian, x86_64).

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33555
Approved-By: Tom Tromey <tom@tromey.com>