removed unused abstraction over paths and value_paths
finish removing parse_value_ident
auto merge of #5255 : jbclements/rust/remove-parse-value-ident, r=graydon

After the removal of the "restricted keyword" feature in 0c82c00dc4f49aeb9b57c92c9a40ae35d8a1ee29 , there's no longer any difference between parse_ident() and parse_value_ident(), and therefore no difference between parse parse_path_without_tps() and parse_value_path().  I've collapsed all of these, removing the redundant functions and eliminating the need for two higher-order arguments.