[clang-format] Remove requirement that port_platform.h is at the top (#36278)

Closes #36278

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36278 from ctiller:port_platform-include-grpc++-impl-clang-format d49bba38bb
PiperOrigin-RevId: 623302426

include/grpc++/impl/.clang-format

@@ -5,9 +5,6 @@ DerivePointerAlignment: false
 PointerAlignment: Left
 IncludeBlocks: Regroup
 IncludeCategories:
-  # port_platform.h is before almost everything
-  - Regex: '^<grpc/(support|impl/codegen)/port_platform.h>'
-    Priority: -100
   # ruby.h is even more first if it's included
   - Regex: '^<ruby/ruby.h>'
     Priority: -200