channel: Don't warn when no CA is set
channel: Don't warn when no CA is set

Since v0.22~19 "Use system-wide trust certificate store", when neither
SpiceSession::ca-file nor SpiceSession::ca are set, the system-wide
CA store will be used to validate the SPICE certificates.
However, there is still a g_warn_if_fail() checking that either ca or
ca-file are set, which causes a runtime warning when trying to use the
system-wide store. This commit removes it.

SPICE/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk