Drop modifications to libsharp

[wthrowe]

I do not believe we are allowed to integrate modified GPL code into
the SpECTRE source. There may be an exception if the modified code
could still be used separately, but the changes we made to libsharp
did not allow that since we replaced its build system with SpECTRE's.

This replaces the contents of the external/libsharp directory with the
unmodified source, and builds it as an external project.

Note to reviewers: in the last commit, the non-libsharp files modified are docs/Installation/Installation.md and external/CMakeLists.txt.

Proposed changes

Upgrade instructions

GNU Make is now a required dependency. Using Ninja to build SpECTRE will continue to work, but it will internally use make to build the bundled libsharp libraries.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsdeppe]

LGTM! Thanks for doing this!!