ci: drop broken uniffi-bindgen pre-install from CI image

#106 added \`cargo install uniffi-bindgen --version "^0.29" --locked\`,
which fails ci-image rebuilds:

  error: could not find \`uniffi-bindgen\` in registry \`crates-io\`
         with version \`^0.29\`

Two errors in the original change:
  1. The crate Gobley installs is \`gobley-uniffi-bindgen\` (its own
     fork on crates.io at 0.3.7), not upstream \`uniffi-bindgen\`.
  2. Gobley installs the binary into \`app/build/gobley-tools-install/
     uniffi-bindgen/\`, not \`~/.cargo/bin\`. A globally pre-installed
     binary wouldn't satisfy the task's UP-TO-DATE check anyway.

\`org.gradle.caching=true\` from #106 already makes
\`installUniffiBindgen\` go UP-TO-DATE on warm runs (verified locally),
so the optimisation is in effect via the build cache instead.

.github/docker/ci/Dockerfile

@@ -53,11 +53,6 @@ RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
     rustup target add aarch64-linux-android && \
     rustup component add rustfmt clippy && \
     cargo install cargo-ndk --locked && \
-    # Pre-built uniffi-bindgen so Gobley's :app:installUniffiBindgen task
-    # finds it ready (instead of running `cargo install uniffi-bindgen` on
-    # every CI build — that took ~50 s of the lsposed/lint job each run).
-    # Version range matches lsposed/native/Cargo.toml's `uniffi = "0.29"`.
-    cargo install uniffi-bindgen --version "^0.29" --locked && \
     chmod -R a+w /usr/local/rustup /usr/local/cargo
 
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