Revert "added leq for fldmap; ty`leq is just an alias for ty`eq"

This reverts commit 721c08d4127eb67d4f64784b36415d929030e9eb.
removed ty/null from clsmap-ty and fldmap2conj
fldmap2conj w/ ty/null seems incorrect, since if a field type is ty/null, then
the field permission will not exist. This will be inconsistent with later
fldmap2perm. I may come back to fix this if in future I found that ty/null
is still needed..
proved letRefTyping-helper/base/unique/L3
cxt-fresh-update-preserves-clsmap-cxt-capmap