revert 1f846616

Details: `RData` is a valid file format for bundling data with an R
package based on the "Writing R Extensions" manual:
http://cran.r-project.org/doc/manuals/r-release/R-exts.html#Data-in-packages

R.gitignore

@@ -4,8 +4,5 @@
 # Example code in package build process
 *-Ex.R
 
-# R data files from past sessions
-.Rdata
-
 # RStudio files
 .Rproj.user/