I claim the ornamental design for a toilet brush holder, as shown and described.
FIG. 1 is a front/right/top perspective view of a toilet brush holder (a front/left/top perspective view would be a mirror image thereof);
FIG. 2 is a rear/left/bottom perspective view thereof (a rear/right/bottom perspective view would be a mirror image thereof);
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof (a right side elevational view would be a mirror image thereof);
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
Some of the broken lines depict portions of the toilet brush holder that form no part of the claimed design. Other broken lines depict environment (e.g., brush handle) that also forms no part of the claimed design.