Remove some pointless code in block.h.

Remove some pointless code in block.h.

There's no visible point in casting the result of a comparison to
bool, because it already is that, at least on C99 compilers.

I see no point in these assertions that a pointer we're about to
dereference isn't null, either.  If it is, the resulting SIGSEGV
will notify us of the problem just fine.

Noted while reviewing Zhihong Yu's patch.  This is basically
cosmetic, so no need for back-patch.

Discussion: https://postgr.es/m/CALNJ-vT9r0DSsAOw9OXVJFxLENoVS_68kJ5x0p44atoYH+H4dg@mail.gmail.com