builtins: update unaccent to leave spaces alone

The previous behavior of unaccent removed spaces, but that is not
compatible with the PG implementation.

Release note (bug fix): The unaccent builtin function no longer removes
spaces.

pkg/sql/logictest/testdata/logic_test/builtin_function

@@ -109,14 +109,16 @@ SELECT unaccent(str) FROM ( VALUES
    ('Żółć'),
    ('⃞h̀ＥＬＬＯ`̀́⃞'),
    ('Softhyphen­separator'),
-   ('some ̂thing')
+   ('some ̂thing'),
+   ('héllö wòrld')
 ) tbl(str)
 ----
 no_special_CHARACTERS1!
 Zolc
 hELLO`
 Softhyphen-separator
-something
+some thing
+hello world
 
 statement error unknown signature: upper\(decimal\)
 SELECT upper(2.2)

pkg/util/unaccent/unaccent.go

@@ -428,7 +428,6 @@ var Dictionary = map[rune]string{
 	'˖':      "+",
 	'˗':      "-",
 	'˜':      "~",
-	'\u0020': "",
 	'\u0027': "",
 	'\u0300': "",
 	'\u0301': "",