[FIX] mail: remove unnecessary css in chatter

This commit removes the css that was creating two layout issues :
[1] When the chatter was rendered under the form view with the
`isInFormSheetBg` class, it added an unwanted background and border.

[2] When an attachment was uploaded and created a preview (eg. in
approvals), it created an unwanted border.

This commit fixes these issues.

task-3334866
Part of task-3326263

closes odoo/odoo#123095

X-original-commit: 5171b0bb2054f4755d0ba70b6820806c5253774d
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/attachments/attachment_view.scss

@@ -4,7 +4,6 @@
         flex: 1 0 auto;
         overflow: hidden;
         width: $o-mail-Chatter-minWidth;
-        border-left: 1px solid map-get($grays, '400');
 
         > .o-mail-Attachment {
             position: relative;

addons/mail/static/src/views/form/form_compiler.js

@@ -112,7 +112,7 @@ patch(FormCompiler.prototype, "mail", {
                 "t-if": `__comp__.hasFileViewer() and __comp__.uiService.size >= ${SIZES.XXL}`,
             });
             const sheetBgChatterContainerHookXml = chatterContainerHookXml.cloneNode(true);
-            sheetBgChatterContainerHookXml.classList.add("o-isInFormSheetBg");
+            sheetBgChatterContainerHookXml.classList.add("o-isInFormSheetBg", "w-auto");
             setAttributes(sheetBgChatterContainerHookXml, {
                 "t-if": `__comp__.hasFileViewer() and __comp__.uiService.size >= ${SIZES.XXL}`,
             });

addons/mail/static/src/views/form/form_renderer.scss

@@ -26,8 +26,8 @@
     }
 
     &.o-isInFormSheetBg {
-        background-color: $white;
-        border-top: 1px solid $border-color;
+        margin-left: calc(var(--formView-sheetBg-padding-x) * -1);
+        margin-right: calc(var(--formView-sheetBg-padding-x) * -1);
     }
 
     &:not(.o-aside):not(.o-full-width) .o-mail-Chatter {