remove -Dmad string length restriction

Under -Dmad, with PERL_MADSKILLS > 0, it prints a warning for every
string that is grown to be >= 1Mb. I can't see that it makes any sense to
do this, so I suspect that this is residual code from MAD development, or
an accidental copying of the almost identical #ifdef HAS_64K_LIMIT
piece of code directly following it.