Remove references to removed sample data

doc/references.rst

@@ -7,7 +7,6 @@ References
 .. [Blakely1995] Blakely, R. (1995). Potential Theory in Gravity and Magnetic Applications. Cambridge: Cambridge University Press. doi:`10.1017/CBO9780511549816 <https://doi.org/10.1017/CBO9780511549816>`__
 .. [Cooper2000] Cooper, G.R.J. (2000), Gridding gravity data using an equivalent layer, Computers & Geosciences, Computers & Geosciences, doi:`10.1016/S0098-3004(99)00089-8 <https://doi.org/10.1016/S0098-3004(99)00089-8>`__
 .. [Dampney1969] Dampney, C. N. G. (1969). The equivalent source technique. Geophysics, 34(1), 39–53. doi:`10.1190/1.1439996 <https://doi.org/10.1190/1.1439996>`__
-.. [Forste_etal2014] Förste, Christoph; Bruinsma, Sean.L.; Abrikosov, Oleg; Lemoine, Jean-Michel; Marty, Jean Charles; Flechtner, Frank; Balmino, G.; Barthelmes, F.; Biancale, R. (2014): EIGEN-6C4 The latest combined global gravity field model including GOCE data up to degree and order 2190 of GFZ Potsdam and GRGS Toulouse. GFZ Data Services. doi:`10.5880/icgem.2015.1 <http://doi.org/10.5880/icgem.2015.1>`__
 .. [Fukushima2020] Fukushima, T. (2020). Speed and accuracy improvements in standard algorithm for prismatic gravitational field. Geophysical Journal International. doi:`10.1093/gji/ggaa240 <https://doi.org/10.1093/gji/ggaa240>`__
 .. [Geosoft1999] Geosoft Incorporated. (1999). Montaj MAGMAP filtering; 2–D frequency domain of potential field data extension for Oasis Montaj v. 6.1.
 .. [Grombein2013] Grombein, T., Seitz, K., Heck, B. (2013), Optimized formulas for the gravitational field of a tesseroid, Journal of Geodesy. doi:`10.1007/s00190-013-0636-1 <https://doi.org/10.1007/s00190-013-0636-1>`__