clk: renesas: rcar-gen3: Remove CLK_TYPE_GEN3_Z2

After recent reworking of Z and Z2 clk handling
CLK_TYPE_GEN3_Z and CLK_TYPE_GEN3_Z2 have come to have precisely
the same meaning. Remove this redundancy by eliminating the latter.

This is not expected to have any run-time effect.

As suggested by Geert Uytterhoeven.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>