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GH-36511: [C++][FlightRPC] Get rid of GRPCPP_PP_INCLUDE (#36679)

### Rationale for this change

It's for gRPC < 1.10 and we require gRPC >= 1.30.0. So we can get rid of it.

### What changes are included in this PR?

Get rid of GRPCPP_PP_INCLUDE.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #36511

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>