Don't strip obsolete tag names from doxygen build errors

**Describe the bug**

Issue #2524 is a doxygen build failure due to an obsolete tag.
Debugging this was more work than necessary because the culprit tag was stripped from the error output by this:
```
# I want to see errors other than:
# Warning: Tag `MAX_DOT_GRAPH_HEIGHT' at line 248 of file Doxyfile has become obsolete.
# To avoid this warning please update your configuration file using "doxygen -u"
string(REGEX REPLACE
"(^|(\r?\n))[^\r\n]*has become obsolete.\r?\n"
"\\1" doxygen_u_error "${doxygen_u_error}")
```
The error output is:
```
CMake Error at /tmp/dynamorio/drmemory/docs/CMake_doxyfile.cmake:256 (message):
/usr/bin/doxygen -u failed: This tag has been removed.
```
whereas it could have been:
```
/usr/bin/doxygen -u failed: warning: Tag 'DOT_TRANSPARENT' at line 243 of file 'Doxyfile' has become obsolete.

This tag has been removed.
```

**To Reproduce**
```
$ make htmldocs
```
**Versions**
- What version of Dr. Memory are you using? git head @bfbc4a118a58a182770d306120cad41b9893c53