version: don't remake semVer if not nil

The SemVer function of a PluginVersion returns the version.Version
instance bound to the PluginVersion.

In former implementations of the structure, this could be nil in some
cases, as only the version components were being registered, and the
code would make sure that they were a valid semver version.

Recent changes reorganised this code by making the semVer attribute
always present, so while theoretically it could be nil, this would
indicate a manipulation error.

Therefore, we don't need to perform this check to re-create the semVer
attribute, and by that change we fix the underlying issue that made
semVer drop its pre-release/metadata parts when doing so.

version/version.go

@@ -123,12 +123,6 @@ func (p *PluginVersion) FormattedVersion() string {
 }
 
 func (p *PluginVersion) SemVer() *version.Version {
-	if p.semVer != nil {
-		// SemVer is an instance of version.Version. This has the secondary
-		// benefit of verifying during tests and init time that our version is a
-		// proper semantic version, which should always be the case.
-		p.semVer = version.Must(version.NewVersion(p.version))
-	}
 	return p.semVer
 }