Remove gaia (#4347)

Gaia is removed from cosmos-sdk repository.

Few changes were required to make sure no packages depend on gaia subpackages.

CI config is amended accordingly.

Unnecessary targets are removed from Makefile.

Simulations run through a lightweight version of gaia renamed to simapp.

Closes: #4104

client/context/errors.go

@@ -17,6 +17,6 @@ Are you sure there has been a transaction involving it?`, addr)
 // height can't be verified. The reason is that the base checkpoint of the certifier is
 // newer than the given height
 func ErrVerifyCommit(height int64) error {
-	return fmt.Errorf(`The height of base truststore in gaia-lite is higher than height %d. 
+	return fmt.Errorf(`The height of base truststore in the light client is higher than height %d. 
 Can't verify blockchain proof at this height. Please set --trust-node to true and try again`, height)
 }