ST9 Maintenance Packs cannot be uninstalled without the original install media.
Why?
Solution
This is as designed. Also the previous MP is needed.
From the "Solid_Edge_ST9_MP*_Readme.txt":
PLEASE NOTE: When uninstalling a MSI Product Update, the original source media and the previous Product Update package, if any, must be available to allow the recovery of previous file versions.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 764SP1
Product: SOLID_EDGE
Application: SETUP
Version: V109.0
Function: INSTALL
Ref: 001-7892211