Remove spvOpcodeIsVariable

Nothing was using it.
diff --git a/source/opcode.cpp b/source/opcode.cpp
index e21c322..8609c02 100644
--- a/source/opcode.cpp
+++ b/source/opcode.cpp
@@ -50,7 +50,7 @@
 #define Instruction(Name,HasResult,HasType,NumLogicalOperands,NumCapabilities,CapabilityRequired,IsVariable,LogicalArgsList) \
   { #Name, \
     Op##Name, \
-    ((IsVariable ? SPV_OPCODE_FLAGS_VARIABLE : 0) | (CapabilityRequired != CapabilityNone ? SPV_OPCODE_FLAGS_CAPABILITIES : 0)), \
+    ((CapabilityRequired != CapabilityNone ? SPV_OPCODE_FLAGS_CAPABILITIES : 0)), \
     uint32_t(CapabilityRequired), \
     0, {}, /* Filled in later. Operand list, including result id and type id, if needed */ \
     HasResult, \
@@ -274,11 +274,6 @@
   return SPV_ERROR_INVALID_LOOKUP;
 }
 
-int32_t spvOpcodeIsVariable(spv_opcode_desc entry) {
-  return SPV_OPCODE_FLAGS_VARIABLE ==
-         (SPV_OPCODE_FLAGS_VARIABLE & entry->flags);
-}
-
 int16_t spvOpcodeResultIdIndex(spv_opcode_desc entry) {
   for (int16_t i = 0; i < entry->numTypes; ++i) {
     if (SPV_OPERAND_TYPE_RESULT_ID == entry->operandTypes[i]) return i;