e4b8deb2 removed the last in-tree uses of PCPU_INC().  Its
potential benefit is also practically nonexistent.  Non-x86
platforms already implement it as PCPU_ADD(..., 1), and according
to [0] there are no recent x86 processors for which the 'inc'
instruction provides a performance benefit over the equivalent
memory-operand form of the 'add' instruction.  The only remaining
benefit of 'inc' is smaller instruction size, which in this case
is inconsequential given the limited number of per-CPU data consumers.

[0]: https://www.agner.org/optimize/instruction_tables.pdf

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D29308