Remove compile-time and runtime checks for SPI in HostInfoMacOSX

There are conditionalized calls to an SPI in HostInfoMacOSX.mm
to test if lldb is being built against a pre-macOS 10.12 SDK,
or being run on a pre-macOS 10.12 system.  macOS 10.12 was released
six years ago, and I don't know of any active users of this system
so let's remove the checks.

Differential Revision: https://reviews.llvm.org/D136900
rdar://101652340