Delete checks and guards for standard C89 headers and assume these are
available: `stdio.h`, `string.h`, `time.h`, `setjmp.h`, `stdlib.h`,
`stddef.h`, `signal.h`.
Some of these we already used unconditionally, some others we only used
for feature checks.
Follow-up to 9c7165e96a3a9a2d0b7059c87c699b5ca8cdae93 #11918 (for `stdio.h` in CMake)
Closes#11940
96c29900bc