pdl: Remove module-level allow(…) attribute

This is in preparation for integration into rootcanal. The rootcanal
code includes the generated code via ‘include!’. This breaks because
of the module-level allow attribute.

Tag: #feature
Bug: 228306436
Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
Change-Id: Ib47eac26c94ede4f51297b096edeee6b43da693d