Remove directory for gem type signatures (#16)

A small follow up to #14 in which I forgot to remove the directory for
gem-related type signatures, which is no longer needed now that the
project is back down to zero dependencies.

Author: brandur

Steepfile

@@ -7,7 +7,6 @@ target :lib do
   library "time"
 
   signature "sig"
-  signature "sig_gem"
 
   configure_code_diagnostics(D::Ruby.strict)
 end