Eliminate the only use of VOIDFLAGS, as part of STMT_START in perl.h

Eliminate the only use of VOIDFLAGS, as part of STMT_START in perl.h

STMT_START has used VOIDFLAGS as part of its conditional compilation since it
was added by commit 728e280303ae3d3b (March 1996). The code originally read:

   /* Now which other defined()s do we need here ??? */

Since then it has been amended to avoid entering that definition for GCC
on Solaris. Given that all current Solaris compilers are C89 conformant,
VOIDFLAGS will always be true (actually 15), so the test is redundant.
Even back in 1996, it's possible that VOIDFLAGS was always non-zero on
SunOS, rendering the test obsolete from the start.

Spotted by Brian Fraser, and extracted from a larger patch of his.