[FIX] *: increase text contrast for cover snippet

*: theme_kea, theme_orchid, theme_nano

The theme customization of the cover snippet was not perfect for those
themes. In particular for the Kea theme where the text was not forced to
white anymore (leaving the default black) and the background filter was
pratically removed, leaving the snippet's text properly contrast with
light brackgrounds but be barely visible with dark backgrounds.

Unfortunately, the Kea theme is the most used currently, alongside the
"Software company" industry... which uses a dark background for the
cover image.

closes odoo/design-themes#526

Signed-off-by: Romain Derie (rde) <rde@odoo.com>

theme_nano/views/snippets/s_cover.xml

@@ -12,7 +12,6 @@
     <xpath expr="//*[hasclass('container')]" position="before">
         <div class="o_we_shape o_web_editor_Bold_12_001 o_we_flip_x"/>
     </xpath>
-    <xpath expr="//*[hasclass('o_we_bg_filter')]" position="replace"/>
     <xpath expr="//*[hasclass('s_parallax_bg')]" position="replace"/>
     <!-- Title & Text -->
     <xpath expr="//h1//font" position="replace" mode="inner">
@@ -24,4 +23,3 @@
 </template>
 
 </odoo>
-