gitweb: remove invalid http-equiv="content-type"

Before this change, gitweb would generate pages which included:

	<meta http-equiv="content-type" content="application/xhtml+xml; charset=utf-8"/>

When a meta's http-equiv equals "content-type", the http-equiv is said
to be in the "Encoding declaration state". According to the HTML
Standard,

	The Encoding declaration state may be used in HTML documents,
	but elements with an http-equiv attribute in that state must not
	be used in XML documents.

	Source: <https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-content-type>

This change removes that meta element since gitweb always generates XML
documents.

Signed-off-by: Jason Yundt <redacted>
Signed-off-by: Junio C Hamano <redacted>