Revert "Merge bitcoin#620: Install headers automatically"

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.

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