This was for Guile < 2.0.9 and we've been requiring 2.0.9+ for some time
already.
* configure.ac: Remove 'GUIX_CHECK_SRFI_37' use and 'INSTALL_SRFI_37'
conditional.
* Makefile.am: Remove code in "if INSTALL_SRFI_37".
(EXTRA_DIST): Remove srfi/srfi-37.scm.in.
* srfi/srfi-37.scm.in: Remove.
* m4/guix.m4 (GUIX_CHECK_SRFI_37): Remove.