Gn: remove redundant prefix setting code.
Gn: remove redundant prefix setting code.

Reuse |android_tool_prefix| in //build/toolchain/gcc_toolchain.gni

Review URL: https://codereview.chromium.org/1727033004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#377965}

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