remove need for redundant class variable

Cython/Compiler/ExprNodes.py

@@ -2992,7 +2992,6 @@ class FormattedValueNode(ExprNode):
     # format_spec     JoinedStrNode or None   Format string passed to __format__
     subexprs = ['value', 'format_spec']
 
-    conversion_chars = 'sra'
     type = py_object_type
     is_temp = True
 

Cython/Compiler/Parsing.py

@@ -1126,7 +1126,7 @@ def p_f_string_expr(s, unicode_value, pos, starting_index):
     expr = p_testlist(scanner)  # TODO is testlist right here?
 
     # validate the conversion char
-    if conversion_char is not None and conversion_char not in ExprNodes.FormattedValueNode.conversion_chars:
+    if conversion_char is not None and not ExprNodes.FormattedValueNode.find_conversion_func(conversion_char):
         s.error("invalid conversion character '%s'" % conversion_char)
 
     # the format spec is itself treated like an f-string