arm64: debug: remove noisy, pointless warning

Sending a SIGTRAP to a user task after execution of a BRK instruction at
EL0 is fundamental to the way in which software breakpoints work and
doesn't deserve a warning to be logged in dmesg. Whilst the warning can
be justified from EL1, do_debug_exception will already do the right thing,
so simply remove the code altogether.

Cc: Sandeepa Prabhu <sandeepa.prabhu@linaro.org>
Reported-by: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>