Revert "arm64: write PID in CONTEXTIDR_EL1 on ctx switch"

arm64_pid_in_contextidr is a bool yet is loaded as a 64-bit word, so
reads out of bounds and will get unknown junk in the upper bits, which
may cause false (the default) to be interpreted as true by the following
cbz. This also doesn't even link depending on the variable's alignment,
currently breaking the clang-12 and clang-13 jobs in GitHub Actions.

Whilst fixing that to use ldrb instead would be trivial, there's a
similar issue reading p_pid below, which is an int32_t yet read as a
64-bit word again. Fixing this means using ldrw or ldrsw, but it's not
clear whether the expectation is for the PID to be zero-extended or
sign-extended.

Revert the commit to fix the build and, if it builds, the run-time
behaviour until it can be corrected and re-landed.

This reverts commit f05795e3f65f305cb770ae91d8e9c8f05d267e0d.