[FIX] html_image_url_extractor: remove `lxml` 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] https://github.com/odoo/odoo/blob/54e58b3e47ee/requirements.txt#L23

html_image_url_extractor/__manifest__.py

@@ -12,6 +12,5 @@
     "license": "AGPL-3",
     "application": False,
     "installable": True,
-    "external_dependencies": {"python": ["lxml"]},
     "depends": ["base"],
 }

requirements.txt

@@ -6,7 +6,6 @@ cryptography==2.6.1
 dataclasses
 dnspython
 josepy
-lxml
 mako
 odoorpc
 openpyxl