qbs build: Remove outdated "minimumQtVersion" specifications.

These were added when the overall project still supported Qt 4. Now the
general requirement is even stronger than what was specified in these
plugins.

Change-Id: I71241e20a72a413d30c4e089bcc19b0762a732da
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

src/plugins/qmlprofiler/qmlprofiler.qbs

@@ -4,7 +4,6 @@ import QtcPlugin
 
 QtcPlugin {
     name: "QmlProfiler"
-    minimumQtVersion: "5.1"
 
     Depends { name: "Qt"; submodules: ["widgets", "network"] }
     Depends { name: "Qt.quick"; condition: product.condition; }

src/plugins/welcome/welcome.qbs

@@ -4,7 +4,6 @@ import QtcPlugin
 
 QtcPlugin {
     name: "Welcome"
-    minimumQtVersion: "5.1"
 
     Depends { name: "Qt"; submodules: ["widgets", "network"] }
     Depends { name: "Qt.quick"; condition: product.condition; }

src/plugins/winrt/winrt.qbs

@@ -4,7 +4,6 @@ import QtcPlugin
 
 QtcPlugin {
     name: "WinRt"
-    minimumQtVersion: "5.0"
 
     Depends { name: "Core" }
     Depends { name: "Debugger" }