[PowerPC] Remove chenzheng1030 from active maintainers (#145519)

chenzheng1030 has left IBM and no longer works on the PowerPC
backend. Move them from active to former maintainers.

Author: nikic

llvm/Maintainers.md

@@ -263,8 +263,6 @@ [email protected] (email), [AlexMaclean](https://github.com/AlexMaclean) (GitH
 
 #### PowerPC backend
 
-Zheng Chen \
-[email protected] (email), [chenzheng1030](https://github.com/chenzheng1030) (GitHub)
 
 #### RISCV backend
 
@@ -522,7 +520,8 @@ Hans Wennborg ([email protected], [zmodem](https://github.com/zmodem)) -- Releas
 Kostya Serebryany ([kcc](https://github.com/kcc)) -- Sanitizers \
 Michael Spencer ([email protected]), [Bigcheese](https://github.com/Bigcheese)) -- Windows support in object tools \
 Alexei Starovoitov ([email protected], [4ast](https://github.com/4ast)) -- BPF backend \
-Evgeniy Stepanov ([eugenis](https://github.com/eugenis)) -- Sanitizers
+Evgeniy Stepanov ([eugenis](https://github.com/eugenis)) -- Sanitizers \
+Zheng Chen ([email protected], [chenzheng1030](https://github.com/chenzheng1030)) -- PowerPC backend
 
 ### Former maintainers of removed components