gnu: nss, nss-certs: Update to 3.31.

Release notes:
<https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/NSS_3.31_release_notes>

* gnu/packages/certs.scm (nss-certs): Update to 3.31.
* gnu/packages/gnuzilla.scm (nss): Likewise.
[source]<patches>: Remove upstream 'nss-disable-long-b64-tests' patch.
[arguments]<#:phases>: Move armhf timeout substitution ...
* gnu/packages/patches/nss-increase-test-timeout.patch: ... here.
* gnu/packages/patches/nss-disable-long-b64-tests.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove it.