Merge bitcoin#626: Revert "Merge bitcoin#620: Install headers automat…

…ically"

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

Pull request description:

  This reverts commit 91fae3a, reversing
  changes made to 5df77a0.

  See discussion in bitcoin-core/secp256k1#625

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

  Closes bitcoin-core/secp256k1#624

Tree-SHA512: 7769ede527ac307bff094603e5974c56b19e41bc2ef55113173d3dbc8e545d6add7ae044753fa0361595e5e7a746d6c8d641f98caa3381b683aa2b61a1742097

Makefile.am

@@ -8,8 +8,6 @@ else
 JNI_LIB =
 endif
 include_HEADERS = include/secp256k1.h
-include_HEADERS += include/secp256k1_ecdh.h
-include_HEADERS += include/secp256k1_recovery.h
 noinst_HEADERS =
 noinst_HEADERS += src/scalar.h
 noinst_HEADERS += src/scalar_4x64.h