[BUGFIX] Avoid usage of subheader in mailform

Since TYPO3 CMS 4.7 the subheader field is rendered by lib.stdheader
if the chosen doctype is HTML 5. However the old mailform (ab)uses this
field for storing the recipient e-mail address. The subheader must
not be displayed in this case.

Resolves: #46094
Releases: 6.2, 6.1, 6.0, 4.7
Change-Id: I19d13e65b941e2514a9c4a12cb37f015661e9d8f
Reviewed-on: https://review.typo3.org/23109
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters