zynq: Remove support for Zynq-7000 SoC

Remove openamp support for Zynq-7000 SoC, following changes are done
1) remove folder apps/machine/zynq7
2) remove folder apps/system/generic/machine/zynq7
3) remove file cmake/platforms/zynq7_generic.cmake
4) remove file cmake/platforms/zynq7_linux.cmake
5) Modify README.md files

Reasons to remove:
1) Support for Zynq-7000 has ended
2) Removing redundant or unmaintained code
3) Reduce technical debt carried by OpenAMP team
4) very few customer using openamp on Zynq-7000 SoC

For using Zynq-7000 SoC support
last working and tested version is (v2023.10)
https://github.com/OpenAMP/open-amp/tree/v2023.10

Signed-off-by: Rajiv Mohan <rajiv.mohan@amd.com>