Remove Solaris build workaround in integration tests

This was introduced in #1821 because of issues with Go 1.9, but that
compiler version is no longer supported.

run_integration_tests.go

@@ -230,13 +230,7 @@ func (env *TravisEnvironment) Prepare() error {
 				"openbsd/386", "openbsd/amd64",
 				"netbsd/386", "netbsd/amd64",
 				"linux/arm", "freebsd/arm",
-				"linux/ppc64le",
-			}
-
-			if os.Getenv("RESTIC_BUILD_SOLARIS") == "0" {
-				msg("Skipping Solaris build\n")
-			} else {
-				env.goxOSArch = append(env.goxOSArch, "solaris/amd64")
+				"linux/ppc64le", "solaris/amd64",
 			}
 		} else {
 			env.goxOSArch = []string{runtime.GOOS + "/" + runtime.GOARCH}