CONTRACTS: Remove assigns.cpp

- Remove the old assigns clause checking implementation
- Remove  assigns.cpp from the Makefile.

Author: Remi Delmas

src/goto-instrument/Makefile

@@ -16,7 +16,6 @@ SRC = accelerate/accelerate.cpp \
       alignment_checks.cpp \
       branch.cpp \
       call_sequences.cpp \
-      contracts/assigns.cpp \
       contracts/contracts.cpp \
       contracts/havoc_assigns_clause_targets.cpp \
       contracts/instrument_spec_assigns.cpp \