[FIX] web,mail,web_tour,test_main_flows,tests: no hardcoded user agent

Main Flow Tour Mobile shouldn't require a specific `user_agent` as it
targets a small screen and not a mobile platform (iOS, Android...).

Actually, during refactoring of the tours (odoo/odoo@3a798039d6),
a confusion was made between the legacy `isMobile`, which represents a
small screen (cf. `env.isSmall`) and `isMobileOS`, which targets
"mobile" platforms (ie. iOS, Android...) independently of the screen size.

This commit applies the proper condition (isSmall) for the tours
management and removes the useless `user_agent` property. It also
removes the logic added to support custom user_agent in the Chrome
automation for testing.

closes odoo/odoo#116186

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>

odoo/tests/common.py

@@ -835,7 +835,6 @@ class ChromeBrowser:
 
         self.window_size = test_class.browser_size
         self.touch_enabled = test_class.touch_enabled
-        self.user_agent = test_class.user_agent
         self.sigxcpu_handler = None
         self._chrome_start()
         self._find_websocket()
@@ -985,9 +984,6 @@ class ChromeBrowser:
             # "'ontouchstart' in window"
             switches['--touch-events'] = ''
 
-        if self.user_agent:
-            switches['--user-agent'] = self.user_agent
-
         cmd = [self.executable]
         cmd += ['%s=%s' % (k, v) if v else k for k, v in switches.items()]
         url = 'about:blank'
@@ -1554,7 +1550,6 @@ class HttpCase(TransactionCase):
     browser_size = '1366x768'
     touch_enabled = False
     allow_end_on_form = False
-    user_agent = None
 
     _logger: logging.Logger = None