build: Fixed overridden compiler PDB settings in VC7 to VC12

The curl tool project files for VC7 to VC12 would override the default
setting with the output filename being the same as the linker PDB file.
As such the compiler file would be overwritten with the linker file
for all debug builds.

To avoid this overwrite and for consistency with the libcurl project
files, removed the setting to force the default filename to be used.