Optimization: remove assertion from SubstMap constructor

A profile [1] reported 3% of a compile run was spent here.

This commit demotes the eager check to -Xdev.

This assertion was added in e557acb9a7.

[1] https://dl.dropboxusercontent.com/u/106552/perf/rudder-nailgun-2014-02-18-1.snapshot

Review by @adriaanm. I was reminded of this inefficiency
while profiling compilation of @fanf's code base.