remove Requirements Set "CustomFunctionsRuntime" from manifest (#124)

The following requirements will cause manifest validation to fail.

  <Requirements>
    <Sets DefaultMinVersion="1.1">
        <Set Name="CustomFunctionsRuntime" MinVersion="1.1"/>
    </Sets>
  </Requirements>

c:\dev\git\Excel-Custom-Functions>npm run validate

> [email protected] validate c:\dev\git\Excel-Custom-Functions
> office-toolbox validate -m manifest.xml

Calling validation service. This might take a moment...
-------------------------------------
Validation: Failed

Error #1:
No supported Office products detected: There are no platforms which fulfil the requirements specified in the manifest. The most common reasons for this failure are issues in the Requirements section. Please review the manifest and try again. (link: undefined)
-------------------------------------

Author: akrantz

manifest.xml

@@ -22,11 +22,6 @@
     <SourceLocation DefaultValue="https://localhost:3000/taskpane.html"/>
   </DefaultSettings>
   <Permissions>ReadWriteDocument</Permissions>
-  <Requirements>
-    <Sets DefaultMinVersion="1.1">
-        <Set Name="CustomFunctionsRuntime" MinVersion="1.1"/>
-    </Sets>
-  </Requirements>  
   <VersionOverrides xmlns="http://schemas.microsoft.com/office/taskpaneappversionoverrides" xsi:type="VersionOverridesV1_0">
     <Hosts>
       <Host xsi:type="Workbook">