ARM: dts: STi: Remove unused clk_ext2f_a9 clock

This clock node is unused, remove it.
This allows to fix the following warnings when compiling
dtb with W=1 option :

arch/arm/boot/dts/stih410-b2120.dtb: Warning (simple_bus_reg):
Node /clocks/clockgen-c0 missing or empty reg/ranges property

arch/arm/boot/dts/stih410-b2260.dtb: Warning (simple_bus_reg):
Node /clocks/clockgen-c0 missing or empty reg/ranges property

arch/arm/boot/dts/stih418-b2199.dtb: Warning (simple_bus_reg):
Node /clocks/clockgen-c0 missing or empty reg/ranges property

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>