delete babel plugin prune-namespace

[samouri]

summary
Removes the prune-namespace plugin originally introduced in #23050 since single-pass is no longer an active pursuit.

Partial for #35573

[amp-owners-bot]

Hey @erwinmombay, @jridgewell! These files were changed:

build-system/babel-plugins/babel-plugin-transform-prune-namespace/index.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-identifier-property/input.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-identifier-property/options.json
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-number-property/input.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-number-property/options.json
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-string-property/input.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/computed-string-property/options.json
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/deep-computed-identifier/input.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/errors/deep-computed-identifier/options.json
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/unused-properties/double-assignment/input.js
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/unused-properties/double-assignment/options.json
build-system/babel-plugins/babel-plugin-transform-prune-namespace/test/fixtures/unused-properties/double-assignment/output.js
+22 more