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.
closesodoo/odoo#116186
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>