lib-ssl-iostream: Remove libdovecot_openssl_common.la

lib-ssl-iostream: Remove libdovecot_openssl_common.la

Using such a library results in the same code being duplicated (in OSX) in
both libssl_iostream_openssl.so and libdcrypt_openssl.so.  This breaks the
idea of openssl_init_refcount, because each one will have their own one.

libdcrypt_openssl.so now links to libssl_iostream_openssl.so, which
shouldn't really be a problem, because lib-ssl-iostream is part of the core
libdovecot.so already. It would have been possible to also install
libdovecot_openssl_common.so and link it to both libssl_iostream_openssl.so
and libdcrypt_openssl.so, but that seems unnecessarily complicated.