stdio: remove erronous assert in dtoa

The value of `k` in dtoa() is an estimate of
floor(log10(d)), where `d` is the number being
converted. The code was asserting that 'k' was
less than 100, but the full range of a double
goes to 10^308 or so.

This means that the majority of the range of
a double would trigger that assert.

sys/src/libstdio/dtoa.c

@@ -811,7 +811,6 @@ dtoa(double _d, int mode, int ndigits, int *decpt, int *sign, char **rve)
 		b5 = -k;
 		s5 = 0;
 	}
-	assert(k < 100);
 	if (mode < 0 || mode > 9)
 		mode = 0;
 	try_quick = 1;