[FIX] letsencrypt: remove `cryptography` from python depends

Such dependency is already included in Odoo requirements using a pinned
version [1]. Adding here could cause to upgrade the library to an
incompatible version.

[1] 710a2b2a7a/requirements.txt (L3)

letsencrypt/__manifest__.py

@@ -17,7 +17,5 @@
     "demo": ["demo/ir_cron.xml"],
     "post_init_hook": "post_init_hook",
     "installable": True,
-    "external_dependencies": {
-        "python": ["acme<2.0.0", "cryptography", "dnspython", "josepy"]
-    },
+    "external_dependencies": {"python": ["acme<2.0.0", "dnspython", "josepy"]},
 }

requirements.txt

@@ -1,7 +1,6 @@
 # generated from manifests external_dependencies
 acme<2.0.0
 astor
-cryptography
 cryptography==2.6.1
 dataclasses
 dnspython