devel/libevent: register CONFLICTS_INSTALL with -devel port

While here, drop needless OPENSSL_{CPP,LD}FLAGS assignments.
With DEFAULT_VERSIONS+=ssl=openssl|libressl, the following
definitions end up in the ${WRKSRC}/Makefile without manual
specification of the paths, as they come from pkgconf(1):

OPENSSL_INCS = -I/usr/local/include
OPENSSL_LIBS = -L/usr/local/lib -lssl -lcrypto

With default OpenSSL implementation (base):

OPENSSL_INCS =
OPENSSL_LIBS = -lssl -lcrypto