LASFileWriter: drop support of proj4 < 4.8
LASFileWriter: drop support of proj4 < 4.8

Motivations:
- makes the code more complicated
- 4.8 was released in March 2012
- we currently use (by default) 4.9.2 which is provided by ParaView
- we were also using 4.9.2 in Paraview 5.1 (before superbuild upgrade)
- I am not aware of any user using proj4 < 4.8 (would implies
compiling their own version and a lot of tweaking)
- support of proj4 < 4.8 is not tested

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