Removed DateTimeImmutable::format() from the CallMap: fully covered by stub

Note: some conditional return type magic was required here.

See: https://github.com/vimeo/psalm/pull/8350#discussion_r937089212

dictionaries/CallMap.php

@@ -1799,7 +1799,6 @@ return [
 'DateTimeImmutable::createFromInterface' => ['self', 'object' => 'DateTimeInterface'],
 'DateTimeImmutable::createFromMutable' => ['static', 'datetime'=>'DateTime'],
 'DateTimeImmutable::diff' => ['DateInterval', 'datetime2'=>'DateTimeInterface', 'absolute='=>'bool'],
-'DateTimeImmutable::format' => ['string', 'format'=>'string'],
 'DateTimeImmutable::getLastErrors' => ['array{warning_count:int,warnings:array<int,string>,error_count:int,errors:array<int,string>}'],
 'DateTimeImmutable::getOffset' => ['int'],
 'DateTimeImmutable::getTimestamp' => ['int|false'],

dictionaries/CallMap_80_delta.php

@@ -41,10 +41,6 @@ return [
       'old' => ['string|false', 'format'=>'string'],
       'new' => ['string', 'format'=>'string'],
     ],
-    'DateTimeImmutable::format' => [
-      'old' => ['string|false', 'format'=>'string'],
-      'new' => ['string', 'format'=>'string'],
-    ],
     'DateTimeZone::listIdentifiers' => [
       'old' => ['list<string>|false', 'timezoneGroup='=>'int', 'countryCode='=>'string|null'],
       'new' => ['list<string>', 'timezoneGroup='=>'int', 'countryCode='=>'string|null'],

dictionaries/CallMap_historical.php

@@ -1060,7 +1060,6 @@ return [
     'DateTimeImmutable::add' => ['static', 'interval'=>'DateInterval'],
     'DateTimeImmutable::createFromMutable' => ['static', 'datetime'=>'DateTime'],
     'DateTimeImmutable::diff' => ['DateInterval', 'datetime2'=>'DateTimeInterface', 'absolute='=>'bool'],
-    'DateTimeImmutable::format' => ['string|false', 'format'=>'string'],
     'DateTimeImmutable::getLastErrors' => ['array{warning_count:int,warnings:array<int,string>,error_count:int,errors:array<int,string>}'],
     'DateTimeImmutable::getOffset' => ['int'],
     'DateTimeImmutable::getTimestamp' => ['int|false'],

stubs/CoreImmutableClasses.phpstub

@@ -12,8 +12,10 @@ class DateTimeImmutable implements DateTimeInterface
 
     /**
      * @psalm-mutation-free
+     *
      * @param string $format
-     * @return string
+     *
+     * @return (\PHP_MAJOR_VERSION is int<0, 7> ? string|false : string)
      */
     public function format($format) {}