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GH-34801: [C++] Remove needless "Requires.private: libcurl openssl" from arrow.pc (#34810)

### Rationale for this change

We need "Requires.prviate: libcurl openssl" in arrow.pc only when we use static AWS SDK for C++ (bundled AWS SDK for C++).

In general, "Requires.private: libcurl openssl" is harmless with system (shared) AWS SDK for C++. But "openssl" is harmful on macOS with Homebrew because OpenSSL from Homebrew isn't installed to `$(brew --prefix)`. It means that users who want to use OpenSSL from Homebrew need to refer OpenSSL by `$(brew --prefix openssl@ 1.1)` or `$(brew --prefix openssl@ 3.0)` explicitly.

So it's helpful that we don't add needless "Requires.private: libcurl openssl" to arrow.pc.

### What changes are included in this PR?

These changes stop adding `Requires.private: libcurl openssl` when we use system AWS SDK for C++.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #34801

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