[mlir][linalg] Retire Linalg ConvOp.

The convolution op is one of the remaining hard coded Linalg operations that have no region attached. It got obsolete due to the OpDSL convolution operations. Removing it allows us to delete specialized code and tests that are not needed for the OpDSL counterparts that rely on the standard code paths.

Test needed due to specialized implementations are removed. Tiling and fusion tests are replaced by variants using linalg.conv_2d.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D111233