Using 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.