Remove string indexing by string (fix #454)

This turns out to have been a bad idea:

    "foo"|.["o"]

it interacts badly with `path()`.

See #454 for the gory details.

tests/all.test

@@ -902,18 +902,6 @@ gsub("(?<x>.)[^a]*"; "+\(.x)-")
 ["fo", "foo", "barfoo", "foobar", "foob"]
 ["fo","","bar","foobar","foob"]
 
-.[","]
-"a,bc,def,ghij,klmno"
-[1,4,8,13]
-
-.[", "]
-"a,bc,def,ghij,klmno"
-[]
-
-.[", "]
-"a,b,, c, d,ef, , ghi, jklmn, o"
-[4,7,13,15,20,27]
-
 [(index(","), rindex(",")), indices(",")]
 "a,bc,def,ghij,klmno"
 [1,13,[1,4,8,13]]