starter: Don't define any hard-coded proposal strings

starter: Don't define any hard-coded proposal strings

Just rely on the default proposals by charon if nothing is defined. The
hard-coded IKE proposal used curve25519, which depends on an optional
plugin (while enabled by default it might still not be loaded, or, like
on Debian, shipped in an optional package). With charon's default
proposal only loaded algorithms are proposed for IKE avoiding this issue.