Revert "Call target_can_do_single_step from maybe_software_singlestep"

Revert "Call target_can_do_single_step from maybe_software_singlestep"

This reverts commit 14de1447c9c52c1bfc52588f8652836f66ac6c47.

An automated tester said that this patch caused a regression on
aarch64:

FAIL: gdb.arch/aarch64-atomic-inst.exp: Step through the ldxr/stxr sequence (timeout)

I looked into it a bit yesterday but couldn't see an obvious problem;
and it's somewhat of a pain to try to debug it at the moment.

Tom de Vries also noticed this and filed it in bugzilla.  So, I'm
backing the patch out until I can port the failing test to the AdaCore
internal test suite in order to find out what went wrong.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28440
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33255