Simplify and clean up pmap_clearbit()
Simplify and clean up pmap_clearbit()

There is no need for calling vm_page_dirty() when clearing "modified" flag as
it is already set for that page in pmap_fault_fixup() or pmap_enter() thanks
to "modified" bit emulation.

Also, there is no need for checking PTE "referenced" or "writeable" flags.  If
there is a request to clear a particular flag we should just do it.

Submitted by:	Zbigniew Bodek <zbb@semihalf.com>
Reviewed by:	gber
Sponsored by:	The FreeBSD Foundation, Semihalf

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