sci-mathematics/agda: allow transformers-0.4 (thanks to Heather and Fernandos)

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Revert "fix mtl RDEPEND major version limit"

The real bug is in transformers part.

This reverts commit 247d80a5112fc711b7b1818ad17e61f705043f89.