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.