package/php-ssh2: fix kconfig circular deps after libssh2 fix

package/php-ssh2: fix kconfig circular deps after libssh2 fix

After we simplified the libssh2 dependencies, the new dependencies were
not propagated to the php-ssh2 package, causing circular dependencies:

    package/openssl/Config.in:1:error: recursive dependency detected!
    package/openssl/Config.in:1:    symbol BR2_PACKAGE_OPENSSL is selected by BR2_PACKAGE_LIBSSH2
    package/libssh2/Config.in:1:    symbol BR2_PACKAGE_LIBSSH2 is selected by BR2_PACKAGE_PHP_SSH2
    package/php-ssh2/Config.in:1:   symbol BR2_PACKAGE_PHP_SSH2 depends on BR2_PACKAGE_OPENSSL

Just drop the now-useless dependency inherited from libssh2: libssh2 now
has no dependency.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>