Move *_DEPENDS together

• Remove ca_root_nss from BUILD_DEPENDS
• Use LIB_DEPENDS for devel/grpc
• Remove libunwind from LIB_DEPENDS

This port itself does not require libunwind.
The dependency was brought by ruby which we do not have to mark it as direct dependency.

Differential Revision: https://reviews.freebsd.org/D15282
Submitted by: sunpoet (myself)
Approved by: mfechner (maintainer)