Remove seld/signal-handler dependency

Author: usox

composer.json

@@ -53,7 +53,6 @@
     "mpscholten/request-parser": "^1.5",
     "php-di/php-di": "^6",
     "scn/phptal": "^3",
-    "seld/signal-handler": "^1.2",
     "slim/http": "^1.2",
     "slim/psr7": "^0.6",
     "slim/slim": "^4.2",