Remove themes_custom.json

The themes which were in themes_custom.json are now in the main
themes.json file. This commit removes the themes_custom.json file,
and updates the Makefile and THEMES.md file to reflect this change.

Makefile

@@ -1,17 +1,14 @@
 themes.json:
+	# this is the url used by https://windowsterminalthemes.dev/
+	# See https://github.com/atomcorp/themes/blob/master/app/src/App.tsx#L18
 	@./scripts/download_theme.sh \
 		https://2zrysvpla9.execute-api.eu-west-2.amazonaws.com/prod/themes \
 		themes
 
-themes_custom.json:
-	@./scripts/download_theme.sh \
-		https://raw.githubusercontent.com/atomcorp/themes/master/app/src/custom-colour-schemes.json \
-		themes_custom
-
 THEMES.md:
 	@go run . themes --markdown 2> THEMES.md
 
-all: themes.json themes_custom.json THEMES.md
+all: themes.json THEMES.md
 	@echo "Running all"
 
 refresh:

themes.go

@@ -121,9 +121,6 @@ func uintPtr(u uint) *uint       { return &u }
 var (
 	//go:embed themes.json
 	themesBts []byte
-
-	//go:embed themes_custom.json
-	customThemesBts []byte
 )
 
 // ThemeNotFoundError is returned when a requested theme is not found.
@@ -146,7 +143,7 @@ func (e ThemeNotFoundError) Error() string {
 // sortedThemeNames returns the names of the themes, sorted.
 func sortedThemeNames() ([]string, error) {
 	var keys []string
-	for _, bts := range [][]byte{themesBts, customThemesBts} {
+	for _, bts := range [][]byte{themesBts} {
 		themes, err := parseThemes(bts)
 		if err != nil {
 			return nil, err
@@ -166,7 +163,7 @@ const distance = 2
 
 // findTheme return the given theme, if it exists.
 func findTheme(name string) (Theme, error) {
-	for _, bts := range [][]byte{themesBts, customThemesBts} {
+	for _, bts := range [][]byte{themesBts} {
 		themes, err := parseThemes(bts)
 		if err != nil {
 			return DefaultTheme, err