Fix compilation with more recent Cabal

It appears that a change in Cabal 3.12 has caused the dummy Lexer.hs
file to no longer be shadowed by the one produced from Lexer.x

Moreover, it no longer seems necessary, since `cabal repl` successfully
loads the project even when Lexer.hs does not exist in the source (only
Lexer.x does)

The simple solution seems to simply delete this hacky Lexer.hs file from
the source.

Author: alt-romes