Remove library/adodb
ADOdb library is now a git submodule

A new 'mantis-1.3' branch was created from upstream's master, which is
currently just before release of V5.19, and will be used to track
MantisBT-specific changes for 1.3.x.

Fixes #14458
Remove library/phpmailer
PHPMailer library is now a git submodule

A new 'mantis' branch was created from upstream's v5.2.6 tag, which will
be used to track MantisBT-specific changes to the library.

Fixes #14458
Revised README.libs after adding git submodules
Updated developers' guide following submodules introduction