Remove unused PulseInternal target

[AgapovOne]

0.19.1 release did not remove PulseInternal completely from Package.swift. It fails to resolve path PulseInternal.

[AgapovOne]

Run cd Pulse && swift build
  cd Pulse && swift build
  shell: /bin/bash -e {0}
  env:
    DEVELOPER_DIR: /Applications/Xcode_12.2.app/Contents/Developer
xcrun: error: missing DEVELOPER_DIR path: /Applications/Xcode_12.2.app/Contents/Developer
Error: Process completed with exit code 1.

Seems to be not an issue with the code :)

[AgapovOne]

https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md

Xcode 12_2 is missing there on macOS-latest.
It should be macOS-11 and /Applications/Xcode_12.5.1.app or /Applications/Xcode_12.5.1.app