arm: dts: k3-am62a-sk: remove GP tiboot3 builds

arm: dts: k3-am62a-sk: remove GP tiboot3 builds

The AM62Ax SoC family was the last part from TI to support the GP
security variant, however this security variant was used mostly
internally and with select early partners and never sold publicly.

To simplify things and to avoid any confusion of which parts are
supported in the future, remove the GP tiboot3.bin builds from binman.

Signed-off-by: Bryan Brattlof <bb@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>