docs: drop unnecessary out-of-frame target

I'm guessing an earlier version of the website used to have the page
contents in <frames>, but this isn't the case anymore so just drop the
unnecessary `target="_main"` :)

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/viewperf.html

@@ -18,8 +18,8 @@
 
 <p>
 This page lists known issues with
-<a href="https://www.spec.org/gwpg/gpc.static/vp11info.html" target="_main">SPEC Viewperf 11</a>
-and <a href="https://www.spec.org/gwpg/gpc.static/vp12info.html" target="_main">SPEC Viewperf 12</a>
+<a href="https://www.spec.org/gwpg/gpc.static/vp11info.html">SPEC Viewperf 11</a>
+and <a href="https://www.spec.org/gwpg/gpc.static/vp12info.html">SPEC Viewperf 12</a>
 when running on Mesa-based drivers.
 </p>
 
@@ -66,13 +66,10 @@ either in Viewperf or the Mesa driver.
 
 <p>
 These tests use features of the
-<a href="https://www.opengl.org/registry/specs/NV/fragment_program2.txt"
-target="_main">
-GL_NV_fragment_program2</a> and
-<a href="https://www.opengl.org/registry/specs/NV/vertex_program3.txt"
-target="_main">
-GL_NV_vertex_program3</a> extensions without checking if the driver supports
-them.
+<a href="https://www.opengl.org/registry/specs/NV/fragment_program2.txt">GL_NV_fragment_program2</a>
+and
+<a href="https://www.opengl.org/registry/specs/NV/vertex_program3.txt">GL_NV_vertex_program3</a>
+extensions without checking if the driver supports them.
 </p>
 <p>
 When Mesa tries to compile the vertex/fragment programs it generates errors
@@ -86,8 +83,8 @@ Subsequent drawing calls become no-ops and the rendering is incorrect.
 
 <p>
 These tests depend on the
-<a href="https://www.opengl.org/registry/specs/NV/primitive_restart.txt"
-target="_main">GL_NV_primitive_restart</a> extension.
+<a href="https://www.opengl.org/registry/specs/NV/primitive_restart.txt">GL_NV_primitive_restart</a>
+extension.
 </p>
 
 <p>
@@ -124,7 +121,7 @@ never specified.
 
 <p>
 A trace captured with
-<a href="https://github.com/apitrace/apitrace" target="_main">API trace</a>
+<a href="https://github.com/apitrace/apitrace">API trace</a>
 shows this sequences of calls like this:
 
 <pre>