[OS/2] Fix build break in xpcom (HashString was not declared in this scope).
Guard more IPC code with MOZ_IPC.

This is needed since IPC is not functioning well on OS/2 so far.
Disable telemetry with MOZ_IPC define as it depends on IPC implementation.

This should make it build on OS/2.
Revert "Fix building of HttpBaseChannel on OS/2."

This reverts commit 42abb3d7c01b1166b37bf55370e6448c07fa1121.