Using [exiftool](http://owl.phy.queensu.ca/~phil/exiftool/), the command would be:  
`exiftool -Orientation= /target/dir/or/File`

Replace `/target/dir/or/File` with the files and/or directories you want to process.

This command creates backup files.  Add `-overwrite_original` to suppress the creation of backup files.  Add `-r` to recurse into subdirectories.  If this command is run under Unix/Mac, reverse any double/single quotes to avoid bash interpretation.