Require sizeof(bool) == 1.

The C standard says that sizeof(bool) is implementation-defined, but we
know of no current systems where it is not 1.  The last known systems
seem to have been Apple macOS/PowerPC 10.5 and Microsoft Visual C++ 4,
both long defunct.

PostgreSQL has always required sizeof(bool) == 1 for the definition of
bool that it used, but previously it would define its own type if the
system-provided bool had a different size.  That was liable to cause
memory layout problems when interacting with system and third-party
libraries on (by now hypothetical) computers with wider _Bool, and now
C23 has introduced a new problem by making bool a built-in datatype
(like C++), so the fallback code doesn't even compile.  We could
probably work around that, but then we'd be writing new untested code
for a computer that doesn't exist.

Instead, delete the unreachable and C23-uncompilable fallback code, and
let existing static assertions fail if the system-provided bool is too
wide.  If we ever get a problem report from a real system, then it will
be time to figure out what to do about it in a way that also works on
modern compilers.

Note on C++: Previously we avoided including <stdbool.h> or trying to
define a new bool type in headers that might be included by C++ code.
These days we might as well just include <stdbool.h> unconditionally:
it should be visible to C++11 but do nothing, just as in C23.  We
already include <stdint.h> without C++ guards in c.h, and that falls
under the same C99-compatibility section of the C++11 standard as
<stdbool.h>, so let's remove the guards here too.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3198438.1731895163%40sss.pgh.pa.us

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