Disable monomial term cache.

This cache has two problems:
- The insufficient key used for the cache lookup causes false cache
  hits. (dolfin issue 63)
- The storing of integrands indefinitely will indirectly store
  references to functions, function spaces, and meshes for the entire
  process lifetime, causing potentially massive memory leaks.
Merge branch 'martinal/fix-dolfin-issue-63'