dev-lang/rust: drop memory requirement check

dev-lang/rust: drop memory requirement check

CHECKREQS_MEMORY should only be used when there's a known
minimum memory requirement to run/build packages with MAKEOPTS=-j1.

If user builds package in parallel it is expected that the build process
will require more memory. However, in most cases we don't know how much
memory is required because we don't know how much memory an additional
job could use at maximum (and if we would know, just add distcc, which
will move memory requirement down to distcc host...).

Otherwise we would cause unnecessarily problems for systems with low
memory but still able to build the package.

Bug: https://bugs.gentoo.org/626134
Package-Manager: Portage-2.3.66, Repoman-2.3.12
Signed-off-by: Thomas Deutschmann <whissi@gentoo.org>