maint: Remove 'cond-expand' forms for Guile 2.0.

maint: Remove 'cond-expand' forms for Guile 2.0.

Note: Leave 'cond-expand' forms used in the build-side modules that can
run on %BOOTSTRAP-GUILE, which is currently Guile 2.0.

* guix/build/compile.scm: Move 'use-modules' clause from 'cond-expand'
to 'define-module' form.
(%default-optimizations): Remove 'cond-expand'.
* guix/build/download.scm (tls-wrap): Remove 'cond-expand'.
* guix/build/syscalls.scm: Remove 'cond-expand' form around
'%set-automatic-finalization-enabled?!' and
'without-automatic-finalization'.
* guix/inferior.scm (port->inferior): Remove 'cond-expand'.
* guix/scripts/pack.scm (wrapped-package)[build]: Remove 'cond-expand'.
* guix/status.scm (build-event-output-port): Remove 'cond-expand'.
* guix/store.scm (open-inet-socket): Remove 'cond-expand'.
* guix/ui.scm (install-locale): Remove 'cond-expand'.
* tests/status.scm ("current-build-output-port, UTF-8 + garbage"):
Remove 'cond-expand'.
* tests/store.scm ("current-build-output-port, UTF-8 + garbage"):
Remove 'cond-expand'.