Merge #626: Revert "Merge #620: Install headers automatically"

662918c Revert "Merge #620: Install headers automatically" (ian)

Pull request description:

  This reverts commit 91fae3ace0291b144b27fd8bbda509042f5400f1, reversing
  changes made to 5df77a0eda6e902a1aa9c6249cdeaec197b1e0cd.

  See discussion in https://github.com/bitcoin-core/secp256k1/pull/625

  After the change, if we enable any module, `make install` fails because of the
  duplicated files in the command line arguments.

  Closes https://github.com/bitcoin-core/secp256k1/issues/624

Tree-SHA512: 7769ede527ac307bff094603e5974c56b19e41bc2ef55113173d3dbc8e545d6add7ae044753fa0361595e5e7a746d6c8d641f98caa3381b683aa2b61a1742097