[CLEANUP] remove dependency on obsolete INTBITS macro

[CLEANUP] remove dependency on obsolete INTBITS macro

The INTBITS macro was found to be already defined on some platforms,
and to equal 32 (while INTBITS was 5 here). Due to pure luck, there
was no declaration conflict, but it's nonetheless a problem to fix.

Looking at the code showed that this macro was only used for left
shifts and nothing else anymore. So the replacement is obvious. The
new macro, BITS_PER_INT is more obviously correct.