MFC (1.74: detach k_rem_pio2f.c from the build since it is now unused).
MFC (1.74: detach k_rem_pio2f.c from the build since it is now unused).

This goes with optimizing and debugging cosf(), sinf() and tanf().  This
is an internal API change.  Hopefully nothing outside of libm abuses
libm's internals.  Especially this internal, since it should never have
existed and had large bugs.

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