[FIX] web_editor: remove custom gradient button from colors inventory

Since [1] when resetting the background color of the header, the footer
or the copyright, the 'menu-custom' property is saved as the 'false'
string.
There is no visual effect of this problem, but in case that property is
used in an SCSS `color-yiq` function call, it will trigger an error.

This commit solves this by removing the root cause of this 'false'
string: during the initialisation of color palettes a list of named
colors is built by browsing the color buttons and associating their
data-color attribute to their background-color, except for buttons
containing a gradient color.
Unfortunately this inventory also looked at the "Custom" gradient
button: this button has no background color (and was thus not detected
as being a gradient and therefore not excluded from the inventory) and
it has data-color="false". It therefore introduced a mapping between ""
and "false" - thus making the reset operation's empty string mapped to
the "false" string. This commit removes that button from the named
colors inventory.

Steps to reproduce:
- select the header
- open the "Colors" palette
- click on the None button (the trashcan icon)
=> the customization properties attachment contained a line
```
'menu-custom': 'false',
```

[1]: https://github.com/odoo/odoo/commit/a48a30f954afcb6ff3a59c4f32b05fd0c2cfcd2b



task-2633169

closes odoo/odoo#84550

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>