dt-bindings: clock: rcar-gen2: Remove obsolete header files

The clock definitions in <dt-bindings/clock/r8a779?-clock.h> were
superseded by those in <dt-bindings/clock/r8a779?-cpg-mssr.h> a long
time ago.

In fact U-Boot never used <dt-bindings/clock/r8a779?-clock.h>.
The unused headers, their replacements, and the corresponding
r8a779?.dtsi files were introduced together in U-Boot v2018.03.

Fixes: 16b6e4aa ("ARM: dts: rmobile: Import R8A7790 DTS from Linux 4.15-rc8")
Fixes: edd15fcf ("ARM: dts: rmobile: Import R8A7791 DTS from Linux 4.15-rc8")
Fixes: a3fb9ff3 ("ARM: dts: rmobile: Import R8A7792 DTS from Linux 4.15-rc8")
Fixes: 92aa0995 ("ARM: dts: rmobile: Import R8A7793 DTS from Linux 4.15-rc8")
Fixes: 9a26fc5a ("ARM: dts: rmobile: Import R8A7794 DTS from Linux 4.15-rc8")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>