[var] Remove HB_OT_TAG_fvar/avar

We're not going to define public macros for tags for tables.
There's little use to those and they are implementation detail.

docs/harfbuzz-sections.txt

@@ -440,8 +440,6 @@ Xhb_ot_layout_lookup_substitute
 
 <SECTION>
 <FILE>hb-ot-var</FILE>
-HB_OT_TAG_avar
-HB_OT_TAG_fvar
 HB_OT_TAG_VAR_AXIS_ITALIC
 HB_OT_TAG_VAR_AXIS_OPTICAL_SIZE
 HB_OT_TAG_VAR_AXIS_SLANT

src/hb-ot-var-avar-table.hh

@@ -88,6 +88,8 @@ struct SegmentMaps : ArrayOf<AxisValueMap>
  * avar — Axis Variations Table
  */
 
+#define HB_OT_TAG_avar HB_TAG('a','v','a','r')
+
 struct avar
 {
   static const hb_tag_t tableTag	= HB_OT_TAG_avar;

src/hb-ot-var-fvar-table.hh

@@ -81,6 +81,8 @@ struct AxisRecord
  * fvar — Font Variations Table
  */
 
+#define HB_OT_TAG_fvar HB_TAG('f','v','a','r')
+
 struct fvar
 {
   static const hb_tag_t tableTag	= HB_OT_TAG_fvar;

src/hb-ot-var.h

@@ -36,9 +36,6 @@
 HB_BEGIN_DECLS
 
 
-#define HB_OT_TAG_fvar HB_TAG('f','v','a','r')
-#define HB_OT_TAG_avar HB_TAG('a','v','a','r')
-
 #define HB_OT_TAG_VAR_AXIS_ITALIC	HB_TAG('i','t','a','l')
 #define HB_OT_TAG_VAR_AXIS_OPTICAL_SIZE	HB_TAG('o','p','s','z')
 #define HB_OT_TAG_VAR_AXIS_SLANT	HB_TAG('s','l','n','t')