Remove envvars.h

We don't use it much, and actually retaining the string literals in the
code makes it easier to grep. There's more benefit to be had from
leaving the literals inside :)

Change-Id: I763b0949254b1633faf48e5454fc2ed11cc806c1
Reviewed-on: http://review.couchbase.org/38778
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>