Merge bitcoin/bitcoin#24738: build: remove faketime unsetting and comments from configure

3d70c05 build: remove faketime unsetting and comments from configure.ac (fanquake)

Pull request description:

  We no-longer use [`faketime`](https://github.com/wolfcw/libfaketime) (it used to be required in gitian), so as far as I'm aware, there is no need for us to unset `FAKETIME` or mention it in our build docs.

ACKs for top commit:
  laanwj:
    Code review ACK 3d70c05
  prusnak:
    Approach ACK 3d70c05
  hebasto:
    ACK 3d70c05, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 9cf89d63b81119f3d2f02975a66ec0b93e861993fdb0e4f70538e3be6e0047dc09ce87ef2de40cbf877647a21706b39ddf07240c77765278d383d7a7878cc7eb

Author: fanquake

configure.ac

@@ -42,14 +42,9 @@ AH_TOP([#ifndef BITCOIN_CONFIG_H])
 AH_TOP([#define BITCOIN_CONFIG_H])
 AH_BOTTOM([#endif //BITCOIN_CONFIG_H])
 
-dnl faketime breaks configure and is only needed for make. Disable it here.
-unset FAKETIME
-
 dnl Automake init set-up and checks
 AM_INIT_AUTOMAKE([1.13 no-define subdir-objects foreign])
 
-dnl faketime messes with timestamps and causes configure to be re-run.
-dnl --disable-maintainer-mode can be used to bypass this.
 AM_MAINTAINER_MODE([enable])
 
 dnl make the compilation flags quiet unless V=1 is used