Drop toc variable in TOML front matters

The `toc` variable was first removed from page & section front matters
in Zola v0.10.0 (2020-02-17).

    getzola/zola@4aa2ba8 (Let toc is visable through Page & Section
      variables in templates (\#818), 2019-10-14)
    https://github.com/getzola/zola/releases/tag/v0.10.0

Then in Zola v0.19.0 (2024-06-21) unknown fields in section front
matter are denied and raise "TOML parse error".

    getzola/zola@c054ed1 (Added support for multiple feeds
      (i.e. generating both Atom and RSS) (\#2477), 2024-06-19)
    https://github.com/getzola/zola/releases/tag/v0.19.0

In total, removal of `toc` variable in front matters solves build
errors with Zola v0.19.0 or newer, and doesn't change the content of
built website (ToC is always collected and whether it's shown is
controlled by templates.)

website/content/manual/_index.md

@@ -1,5 +1,4 @@
 +++
 title = "Manual"
 sort_by = "weight"
-toc = true
 +++

website/content/news/2021-02-26-Release-v6.md

@@ -1,7 +1,6 @@
 +++
 title = "arara version 6 released"
 description = "arara version 6 released"
-toc = true
 +++
 
 After approximately one year, we are proud to announce the release of arara

website/content/news/2022-05-01-Release-v7.md

@@ -1,7 +1,6 @@
 +++
 title = "arara version 7 released"
 description = "arara version 7 released"
-toc = true
 +++
 
 After more than one year of on-and-off work, we are proud to announce the

website/content/news/_index.md

@@ -1,5 +1,4 @@
 +++
 title = "News"
 sort_by = "date"
-toc = false
 +++