Remove FromIrep module, as it is not used and has no tests (rust-lang#619)

Author: danielsn

compiler/cbmc/src/irep/from_irep.rs

@@ -14,7 +14,6 @@
 //! TODO: Parser for json symbol tables into the internal irep format
 //! TODO: Investigate memory usage, and consider using sharing to reduce memory usage
 
-mod from_irep;
 mod irep;
 mod irep_id;
 mod symbol;