Remove parse_execution_skipped

This was unused.

src/parse_execution.h

@@ -21,9 +21,6 @@ enum parse_execution_result_t {
     parse_execution_errored,
     /// The job was cancelled (e.g. Ctrl-C).
     parse_execution_cancelled,
-    /// The job was skipped (e.g. due to a not-taken 'and' command). This is a special return
-    /// allowed only from the populate functions, not the run functions.
-    parse_execution_skipped
 };
 
 class parse_execution_context_t {

src/parser.cpp

@@ -686,8 +686,6 @@ eval_result_t parser_t::eval_node(parsed_source_ref_t ps, tnode_t<T> node, block
             return eval_result_t::error;
         case parse_execution_cancelled:
             return eval_result_t::cancelled;
-        case parse_execution_skipped:
-            DIE("skipped should not be returned from run functions");
     }
 }