Curves: Add remove_selection function

This adds a `remove_selection` function that can be used by other
objects that make use of `CurvesGeometry`.

@@ -1115,3 +1100,1 @@
-        BLI_assert_unreachable();
-    }
-    if (attributes.domain_size(domain) != domain_size_orig) {
+

@@ -188,0 +189,4 @@
+/** \name Editing
+ * \{ */
+
+bool remove_selection(bke::CurvesGeometry &curves, eAttrDomain selection_domain);