Remove incorrect clock warning on Mem.read (backport #2721) (#2722)

* Remove incorrect clock warning on Mem.read (#2721)

Mem.read is combinational and thus unaffected by the clock, and so it
does not make sense to issue warnings about the current clock in this
context.

(cherry picked from commit 5fdf74f9

)

* Keep old version of MemBase.clockWarning for binary compatibility

This method is impossible for users to call, but it is easy enough to
keep around a version of it to make MiMa happy.

Co-authored-by: Andrew Waterman <andrew@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>